The present invention relates generally to magnetic resonance imaging (MRI), and more particularly, to a method and apparatus to acquire cardiac-gated images in near-single-breath-hold times.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or xe2x80x9clongitudinal magnetizationxe2x80x9d, MZ, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (GxGy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
In imaging the heart, one has to contend with both respiratory motion and cardiac motion. The former being best controlled using a breath-held technique or some manner of respiratory compensation. Single-shot magnetic resonance imaging using Echo Planar Imaging (EPI) techniques are able to acquire an image in 50-100 msec, thereby eliminating cardiac motion artifacts, but result in low spatial resolution and image signal-to-noise ratio. Moreover, it is well known in the art that single-shot EPI acquisitions (including single shot spiral acquisitions) suffer from off-resonance effects which is manifested by either spatial distortion (with rectilinear read-out) or spatial blurring (with spiral acquisitions).
Spatial resolution and image signal-to-noise ratio (S/N) is restored by segmenting the acquisition over several cardiac cycles. In order to minimize the image blurring as the result of cardiac motion over several cardiac cycles, the segmented acquisition approach gates data acquisition such that data for the desired image is acquired over a small temporal window within each cardiac cycle and gated such that the acquisition occurs at the same phase of the cardiac cycle over subsequent acquisitions. The partitioning or segmentation of the data acquisition over several cardiac cycles is often referred to as a segmented k-space acquisition.
Such acquisition techniques yield images with high image signal-to-noise ratio and high spatial resolution. By keeping the data acquisition window within each cardiac cycle short, cardiac motion blurring over this temporal window is minimized. However, the trade-off here is that a smaller acquisition window implies greater segmentation where all necessary data required to reconstruct an image is spread out over a larger number of cardiac cycles and increases the breath-hold period (scan time). With two-dimensional image acquisition using gated segmented k-space techniques, acquisition windows of between 50-100 msec have been used for scan times of between 12-20 seconds.
Obviously, with three-dimensional imaging, the amount of data is substantially increased due to the need to spatially encode for the third slice direction. Hence, for images at the same in-plane spatial resolution as in a two-dimensional acquisition, the total scan time is increased by a factor equal to the number of slice partitions in the three-dimensional volume. As a result, using the same acquisition parameters as the two-dimensional acquisition renders the scan time of a three-dimensional acquisition to exceed a single breath-hold time for a typical patient suffering from cardio-vascular disease.
In current three-dimensional cardiac imaging, due to the longer scan times, data acquisition is either respiratory-gated or breath-held using segmented echo planar imaging (EPI). If respiratory-gated, 3D CINE images are acquired over several minutes, and the quality of the data acquisition is dependent on the patient maintaining a relatively stable respiration pattern over a period of 6-10 minutes. The current state-of-the-art in breath-held 3D acquisitions is often characterized by low spatial resolution and only a single phase of the cardiac cycle is acquired. The acquisition period has been reported to be between 20 and 40 seconds. Volumetric imaging is accomplished by acquiring data over several different breath-hold periods and combining the data acquisitions. However, after reconstructing images with data acquired over different breath-hold periods, temporal and spatial discrepancies and inaccuracies can occur, resulting in images that are not well defined and/or blurred. Moreover, in order to attain these shorter scan times, the acquisition window in the current 3D acquisitions are often long. Thus, the need to accommodate a shorter breath-hold period leads to increased spatial blurring from cardiac motion as a direct consequence of a larger data acquisition window within each cardiac cycle.
In addition, respiratory-gated techniques using navigator echoes for monitoring the respiratory motion do not lend themselves to a multi-phase or CINE acquisition as a separate pulse sequence section must also be played out within each cardiac interval to interrogate the displacement of the diaphragm. Furthermore, the acquisition of data for the different cardiac phases may not necessarily be at the same respiratory phase, leading to mixed image quality where some phase images closer to when the navigator echo segment was executed having better image quality than that more distant in time.
Some uses of gated acquisitions are in the evaluation of cardiac wall mass and ventricular ejection fractions. With current two-dimensional acquisition schemes, several parallel two-dimensional acquisitions covering the heart are made. From this data set, a simple integration scheme (e.g., Simpson""s rule) is used to calculate volumetric data in the third dimension. Moreover, the two-dimensional acquisition techniques require that each slice be obtained in a single breath-hold. Therefore, to cover the entire heart in a short-access view, between 8-12 breath-holds are needed. Faster 2D techniques, using EPI, increases the acquisition time, but image quality suffers. In addition, the patient may be at a different breath-hold position for each acquisition and this can lead to error in the final measurements.
It would therefore be advantageous to have a technique in which a fast 3D acquisition could be accomplished in a single breath-hold.
The present invention relates to a technique for acquiring cardiac MR images in a time at least equivalent to a typical breath-hold using a variable temporal k-space sampling technique that solves the aforementioned problems. This fast, single breath-hold 3D acquisition can not only be used in a CINE (multi-phase) acquisition for the diagnosis of cardiac wall motion abnormalities, evaluation of ventricular end-diastolic and end-systolic volumes, but also in a single phase mode with magnetization preparation (such as an inversion recovery rf pulse) for the evaluation of myocardial infarction or in coronary artery angiography.
The present invention includes a three-dimensional acquisition technique that employs variable temporal sampling of 3D k-space to produce volume images of the heart within a reasonable breath-hold period. By performing an ECG-gated 3D single phase or multi-phase acquisition (3D CINE) of the heart using fast gradient-recalled echo (GRE) or steady-state free-precession (SSFP-FIESTA) pulse sequences, volumetric images can be generated during a time equivalent to a reasonable single breath-hold with minimal temporal and spatial discrepancies or inaccuracies as compared to images acquired over several different breath-hold periods.
In accordance with one aspect of the invention, a method of acquiring MR images includes partitioning MR data acquisition into at least low and high spatial frequency view partitions, and segmenting MR data acquisition in each of the partitions. The method includes acquiring multiple segments of MR data from each phase of a cardiac cycle in the low spatial frequency view partition, and acquiring multiple segments of MR data from each phase of the cardiac cycle in the high spatial frequency view partitions. Preferably, the acquisition of MR data in the low spatial frequency view partitions is performed relatively more often as compared to the acquisition of MR data in the high spatial frequency view partition. This permits a smaller acquisition window during the acquisition of the low spatial frequency data to minimize cardiac motion blurring artifacts. An MR image is then reconstructed with the MR data acquired from each of the low and high spatial frequency view partitions with reduced temporal and spatial inaccuracies.
In accordance with other aspects of the invention, a computer system and a computer program are disclosed for use with imaging apparatus to acquire data and images. The data acquisition is partitioned into low and high spatial frequency view partitions, and optionally additionally partitioned into an intermediate spatial frequency. The spatial frequency data can be partitioned either along the phase encoding (ky) direction, or both the phase encoding (ky) and slice encoding (kz) direction. In the latter case, the segmentation would be in the radial spatial frequency in the ky-kz plane. MR data acquisition is segmented in each of the partitions such that multiple segments of data are acquired in each phase of a cardiac cycle in the low and high spatial frequency view partitions. An MR image is reconstructed with the MR data acquired from each of the low and high spatial frequency view partitions.
By using such a variable temporal k-space sampling technique, these gated 3D acquisitions can produce volumetric images of the heart within a reasonable breath-hold period. Cardiac images can be acquired at both diastole and systole to thereby facilitate a fast, single breath-hold technique for the measurement of ventricular volumes and ejection fractions without the discrepancies and inaccuracies that typically occur when images are acquired over several different breath-hold periods.
Various other features, objects and advantages of the present invention will be made apparent from the following detailed description and the drawings.